Jose Mourinho's side will visit Spain on May 4 for the first leg before the second leg at Old Trafford the following week.
Manchester United reached the last four with an extra-time win over Belgian side Anderlecht yesterday.
The Premier League side are favourites to win the competition, which would guarantee qualification for next season's Champions League.
In the second semi-final, Ajax will play Lyon.
Marcos Rojo snapped arriving for training with Man United team-mates Fri, April 21, 2017
Marcos Rojo has been pictured driving himself to training a day after being stretchered off during Manchester United's win over Anderlecht
Play slideshow Cavendish Press 1 of 11
Manchester United boss Jose MourinhoWhen will the semi-finals be played?
The first leg will be played on Thursday May 4, with the second leg on May 11.
When is the Europa League final?
The Europa League final takes place on May 24 (19:45 BST) at the Friends Arena in Stockholm, Sweden.